JPH0638491B2 - Field effect transistor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a field effect transistor (hereinafter abbreviated as FET element) using an organic semiconductor.

[Conventional technology]

A π-conjugated polymer has a chemical structure consisting of conjugated double bonds and conjugated triple bonds, and is a band composed of a valence band and a conduction band formed by overlapping π-electron orbitals and a forbidden band separating them. It is believed to have a structure. The band gap varies depending on the material, but is in the range of 1.5 to 4 eV for most π-conjugated polymers. For this reason, many π-conjugated polymers are insulators by themselves. However, it is possible to remove electrons from the valence band (oxidation) by chemical methods, electrochemical methods, physical methods, etc.
Alternatively, injecting (reducing) electrons into the conduction band (hereinafter,
It is briefly described that a carrier carrying an electric charge is generated by (called doping). As a result, the conductivity can be varied over a wide range from the insulator region to the metal region by controlling the amount of doping. The polymer obtained when the doping reaction is an oxidation reaction is p-type, and when the doping reaction is a reduction reaction, it is n-type. This is similar to the case of adding impurities in an inorganic semiconductor. Therefore, a semiconductor element using a π-conjugated polymer as a semiconductor material can be manufactured.

Specifically, a Schottky junction device using polyacetylene (Journal of Applied Physics (J.Appl.Phys.) 52, 869, 1981, JP-A-56-147)
486) and Schottky type junction devices using polypyrrole-based conjugated polymers (Japanese Patent Laid-Open No. 59-63760 etc.). Further, a heterojunction device in which n-CdS, which is an inorganic semiconductor, and p-type polyacetylene are combined has been reported (J. Appl. Phys, 51, 4252, 1980). A pn homojunction element using p-type and n-type polyacetylene is known as a junction element in which π-conjugated polymers are combined (Applied Physics Letters (Ap
pl.Phys.Lett.) Vol. 33, p. 18, 1978). Further, a heterojunction element composed of polyacetylene and poly (N-methylpyrrole) has been reported (J. Appl. Phys. 58, 1279).
P., 1985).

On the other hand, an FET using a π-conjugated polymer as a semiconductor layer
As the element, polyacetylene (J.Appl.Phys.54, 325
P. 5, 1983) and poly (N-methylpyrrole)
(Polymer Preprints Japan
s, Japan) 34, No. 4, page 917, 1985).

FIG. 2 is a cross-sectional view of a conventional FET device using polyacetylene.

In the figure, 1 is glass serving as a substrate, 2 is an aluminum film serving as a gate electrode, 3 is a polysiloxane film serving as an insulating film, 4 is a polyacetylene film serving as a semiconductor layer, and 5 and 6 are source and drain electrodes, respectively. It is a gold film.

Next, the operation will be described. When a voltage is applied between the source electrode 5 and the drain electrode 6, a current flows between the source electrode 5 and the drain electrode 6 through the polyacetylene film 4. At this time, if a voltage is applied to the gate electrode 2 provided on the glass substrate 1 and separated from the polyacetylene film 4 by the insulating film 3, the electric conductivity of the polyacetylene film 4 can be changed by the electric field effect, so that the source-drain can be changed. Current can be controlled. It is considered that this is because the width of the depletion layer in the polyacetylene film 4 adjacent to the insulating film 3 changes depending on the voltage applied to the gate electrode 2 and the effective channel cross-sectional area of holes changes. But,
In this FET device, the stability of the device itself is extremely poor because the polyacetylene itself is rapidly deteriorated by oxygen and moisture in the air rather than the problem of the device characteristics.

FIG. 3 shows a cross-sectional view of an FET element having poly (N-methylpyrrole) as a semiconductor layer. In the figure, 3 is silicon oxide which serves as an insulating film, and 4 is poly (N) which functions as a semiconductor layer.
-Methylpyrrole) films 5 and 6 are gold films serving as a source electrode and a drain electrode, respectively, and 7 is p-type silicon serving as a substrate and a gate electrode. Also in this case, the current (electric conductivity) flowing between the source electrode 5 and the drain electrode 6 through the semiconductor layer 4 can be controlled by the voltage applied to the gate electrode.

[Problems to be solved by the invention]

However, in the case where the π-conjugated polymer film such as polyacetylene or poly (N-methylpyrrole) is used only in the semiconductor layer of the FET element, the conductivity between the source and the drain is greatly increased depending on the voltage applied from the gate. It cannot be changed, and there has been a demand for improvement in characteristics from a practical viewpoint.

The present invention has been made in order to solve the above problems, and obtains a FET element which operates stably and can greatly change the conductivity between the source and the drain by the voltage applied from the gate. With the goal.

[Means for solving problems]

In the FET element according to the present invention, one of the source and the drain is composed of the first π-conjugated polymer film, and the semiconductor layer which is the current path is formed into the first π-conjugated polymer film. Are composed of different second π-conjugated polymer films.

[Action]

In the present invention, by using the π-conjugated polymer for both of the source and the drain of the FET element and the semiconductor layer that is the current passage, only π-conjugated polymer is used for the conventional semiconductor layer that is the current passage. -The FET element can be operated with significantly superior characteristics to the conventional element as compared with the case of using a conjugated polymer.

〔Example〕

FIG. 1 shows an example of a configuration diagram of the FET element of the present invention.
In the figure, 1 is a substrate, 2 is a metal film provided as a gate electrode on the substrate 1, 3 is an insulating film, 4 is a π-conjugated polymer film that serves as a semiconductor layer, and 8 is a lead from a source. A metal film acting as a line, 9 is a π-conjugated polymer film acting as a source, and 10 is a metal film acting as a drain. The above is the case where the π-conjugated polymer film is used as the source. However, when the π-conjugated polymer film is used as the drain, 8 is a metal film that functions as a lead wire from the drain, and 9 is the drain. The used π-conjugated polymer film 10 serves as a metal film serving as a source.

Here, the materials used in the present invention are as follows.

Any material can be used for the substrate 1 as long as it is an insulating material, and specifically, various insulating plastics such as glass, an alumina sintered body, a polyimide film, and a polyester film can be used. Metal film 2 acting as a gate electrode, metal film 8 acting as a lead wire from a source or drain, metal film 1 acting as a drain or source
As 0, metals such as gold, platinum, chromium, palladium, aluminum, and indium, tin oxide, indium oxide, indium tin oxide (ITO), and the like are generally used. Of course, these materials are used. The material is not limited to the above, and two or more kinds of these materials may be used. Here, as the method for providing the metal film, there are methods such as vapor deposition, sputtering, plating, and CVD growth.

Further, it is practically preferable that the metal films 8 and 10 are in ohmic contact with the π-conjugated polymer films 9 and 4, respectively.

In the FET element of the present invention shown in FIG. 1, p-type silicon or n-type silicon can be used as both the gate electrode 2 and the substrate 1. In this case, the substrate 1 can be omitted. Further, in this case, the volume resistivity of p-type silicon or n-type silicon is π− used as the semiconductor layer.
It is practically preferable that the size is smaller than that of the conjugated polymer. Furthermore, a conductive organic polymer may be used as the gate electrode. It is also possible to use a metal plate such as a stainless steel plate or a copper plate, which doubles as the gate electrode 2 and the substrate 1, depending on the purpose of use.

As the insulating film 3, any inorganic organic material can be used as long as it is insulative. Generally, silicon oxide (SiO ₂ ), silicon nitride, aluminum oxide, polyethylene, polyvinylcarbazole, polyphenylene sulfide. , Polyparaxylene, etc. are used. As a method for producing these insulating films, there are a CVD method, a plasma CVD method, a vapor deposition method, a spin coating method, a cluster ion beam vapor deposition method and the like, and any method can be used. Furthermore, the LB single molecule accumulation method can also be used. When p-type silicon or n-type silicon is used as the gate electrode 2 and the substrate 1, a silicon oxide film obtained by a thermal oxidation method of silicon or the like is preferably used as the insulating film 3.

As the π-conjugated polymer used in the present invention, any π-conjugated polymer can be used. Specifically, polypyrrole, poly (N-substituted pyrrole), poly (3,4-
Di-substituted pyrrole), poly (3-substituted pyrrole), polythiophene, poly-3-substituted thiophene), poly (3,4-
Disubstituted thiophene), polyaniline, polyazulene, polypyrene, polycarbazole, poly (N-substituted carbazole), polyselenophene, polyfuran, polybenzothiophene, poly (phenylene vinylene), polybenzofuran, poly (paraphenylene), polyindole, Examples thereof include polyisothionaphthene, polypyridazine, polydiacetylenes, and graphite polymers, but are not limited to these. However, the FET characteristics,
Π- having a five-membered heterocyclic ring for film-forming properties and ease of synthesis
Conjugated polymers are often used, among which the general formula (However, X is one of S and O atoms, R ₁ and R ₂ are —H, CH ₃ , —OCH ₃ , —C ₂ H ₅ and —.
One of OC ₂ H ₅ groups, n is an integer), and a general formula (However, R ₁ and R ₂ are —H, —CH ₃ , —OCH.
_{3, -C} 2 one of a _{H 5} and _-OC 2 _{H 5} group, _{R 3} is _{-H, -CH 3, -C 2 H} 5, -C 3 H 7, and One of the groups, n is an integer. ) Are particularly preferred, and polythiophene, poly (3-methylthiophene), polypyrrole, and poly (N-methylpyrrole) are frequently used from the practical viewpoint.

As a method for producing these π-conjugated polymer films, a π-conjugated polymer obtained by an ordinary polymer synthesis method is provided by spin coating, vapor deposition, dipping, etc., or a catalyst is applied in advance. By the way, there are a method of obtaining by introducing a monomer gas, a CVD method, a photo CVD method, a chemical oxidative polymerization method, an electrochemical polymerization method, and the like, but the method is not limited to these. It is also possible to use the LB method in which the monomer is spread on a subphase such as water or glycerin to form a monomolecular film or a cumulative film and deposited on the substrate. At this time, the π-conjugated polymer film can be obtained by a method of polymerizing before being deposited on the substrate or a method of polymerizing after being deposited. However, the electrochemical polymerization method is preferably used from the viewpoints of film-forming property and ease of production.

The π-conjugated polymer has a low electric conductivity without being subjected to a doping treatment, but generally has a property as a p-type semiconductor. However, a doping process is often performed to improve the characteristics of the FET device. There are chemical methods and physical methods for this doping (industrial materials, 34, No. 4, p. 55, 1986). The former includes methods such as (i) doping from the gas phase, (ii) doping from the liquid phase, (iii) electrochemical doping and (iv) photoinitiated doping, and the latter includes ion implantation.
Both can be used. However, the electrochemical doping method is preferably used from the viewpoint of operability and controllability of the doping amount. Moreover, in electrochemical doping,
When the π-conjugated polymer is obtained by the electrochemical polymerization method, it has an advantage that the doping amount can be controlled by the same device after the polymerization.

As an example, a method of forming a π-conjugated polymer film by an electrochemical polymerization method will be described. In the electrochemical polymerization method, a monomer corresponding to a π-conjugated polymer and a supporting electrolyte are dissolved in an organic solvent or water or a mixed solvent of water and an organic solvent to prepare a reaction solution. In the fabrication of the FET device of the present invention shown in FIG. 1, a metal film 8 serving as a source or drain lead wire is used as a working electrode to cause a polymerization reaction by causing a current to flow between it and a counter electrode such as platinum,
A desired π-conjugated polymer film 9 acting as a source or drain is deposited on the metal film 8 acting as a source or drain lead wire.

Next, using a reaction solution containing a monomer corresponding to a π-conjugated polymer different from the π-conjugated polymer film 9 and a supporting electrolyte, at least one of the π-conjugated polymer film 9 and the metal film 10 is used. Electrochemical polymerization is performed as a working electrode, and a desired π is formed on and between the π-conjugated polymer film 9 and the metal film 10.
-Coating with the conjugated polymer film 4. Since the anion of the supporting electrolyte is generally doped in the π-conjugated polymer synthesized by the electrochemical polymerization method, the doping amount may be adjusted for the purpose of obtaining excellent characteristics as an FET element. Generally, due to the characteristics of the FET device, at least one of the π-conjugated polymer films 4 and 9 is doped, and the various doping methods described above are used depending on the structure of the device.

Any organic solvent may be used as the organic solvent in the electrochemical polymerization method as long as it can dissolve the supporting electrolyte and the above-mentioned monomer. (DM
A polar solvent such as SO), dichloromethane, tetrahydrofuran, ethyl alcohol and methyl alcohol is used alone or as a mixed solvent of two or more kinds. The supporting electrolyte has a high oxidation potential and a high reduction potential, and does not undergo oxidation or reduction reaction itself during electrolytic polymerization, and is a substance capable of imparting conductivity to a solution by being dissolved in a solvent, for example, Perchloric acid tetraalkylammonium salt, tetraalkylammonium tetrafluoroborate salt, tetraalkylammonium hexafluorophosphate salt, tetraalkylammonium paratoluenesulfonate salt, sodium hydroxide and the like are used, but of course two or more kinds may be used in combination. I do not care.

In the FET element of FIG. 1 which is an embodiment of the present invention, the location where all π-conjugated polymer films are produced by the electrochemical polymerization method has been described above. However, depending on the structure of the FET element, The FET element can be manufactured by using the electrochemical polymerization method in combination with another film forming method or by only using another film forming method. The FET element of the present invention thus obtained is useful as a driving circuit for a switching element or a large area liquid crystal display element.

〔Concrete example〕

Hereinafter, details of the present invention will be described with reference to specific examples, but of course, the present invention is not limited to these specific examples.

Concrete Example 1 An n-type silicon plate (3.0 cm × 3.0 cm) having a conductivity of 6 S / cm and a thickness of 380 μm was provided with a silicon oxide film having a thickness of about 3000 Å on both sides by a thermal oxidation method. Next, using a positive photoresist on one side, draw a pattern (each effective area: 0.2 cm × 0.8 cm; distance between both patterns: 6 μm) for forming a metal film that works as a source or drain and a drain or source lead wire. After that, 200 Å of chrome film is provided by vacuum evaporation method,
After forming a gold film 300 Å on it, remove the resist to act as the source lead wire and drain, or
Alternatively, a gold film that functions as a drain lead wire and a source is formed. Lead wires were further formed on the gold films with silver paste, and the contact portions were fixed with epoxy resin to obtain an element substrate.

A solution of tetramethylammonium and p-toluenesulfonate (0.7 g) as an electrolyte in 100 ml of acetonitrile was aerated with a nitrogen gas filter for about 40 minutes to completely dissolve the electrolyte, and then 0.4 ml of pyrrole was added. The product was used as a reaction solution. One of the gold films on the silicon plate was used as a working electrode, a platinum plate (1 cm × 2 cm) was used as a counter electrode, and an SCE (saturated calomel electrode) was used as a reference electrode, and these were immersed in a reaction solution. A constant current (30 μA) was passed between the counter electrode and the working electrode for 6 minutes under a nitrogen gas flow to deposit polypyrrole serving as a source or a drain only on the working electrode. After the synthesis, the substrate was left in an open circuit state for about 15 minutes, then the substrate on which polypyrrole was adhered was taken out from the reaction solution, washed twice with acetonitrile that had been deoxygenated beforehand, dried by blowing nitrogen gas, and then put in a vacuum. saved.

After dissolving tetraethylammonium perchlorate (0.7 g) and 2,2'-dithiophene (0.4 g) as an electrolyte in 100 ml of acetonitrile, nitrogen gas was bubbled for about 30 minutes to prepare a reaction solution. A source polypyrrole film and a drain gold film or a drain polypolyrole film and a source gold film are simultaneously used as working electrodes on the above-mentioned substrate on which polypyrrole is coated, and a platinum plate (1 cm x 2 cm) is used as a counter electrode. Were immersed in the reaction solution using SCE as the reference electrode. Under a nitrogen gas stream, first, 1V was applied to the SCE with a potentiostat to the working electrode for 1 minute, and then a constant current (30 μA) was passed between the working electrode and the counter electrode for 5 minutes to form the working electrode. A polythiophene film serving as a semiconductor layer was deposited on the polypyrrole film and the gold film and on the silicon oxide between them.

Next, the potential of the working electrode was set to 0 V with respect to SCE by a potentiostat for 4 hours to adjust the doping amount of the polypyrrole film and the polythiophene film. Then, it was washed twice with acetonitrile that had been deoxygenated in advance, dried by blowing nitrogen gas, and then completely dried in vacuum.

The silicon oxide on the other surface of the silicon plate not covered by the polypyrrole film and the polythiophene film, which are π-conjugated polymers provided as described above, was partially (about 0.5.
cm ² ), make an ohmic contact with n-type silicon with an indium-gallium alloy, take out a lead wire from this, fix the contact part with epoxy resin, and n-type silicon acts as a gate electrode through this lead wire I decided to do it.

As described above, the FET device of the embodiment of the present invention having the structure shown in FIG. 1 was manufactured as a prototype. In this specific example, 1 in FIG.
And 2 are composed of n-type silicon, are a substrate and a gate electrode, 3 is a silicon oxide that acts as an insulating film, 4 is a polythiophene film that is a semiconductor layer, 9 is a polypyrrole film that acts as a source or drain, and 8 is a source or drain. A chromium film coated with a gold film serving as a lead wire from No. 10 is a chromium film coated with a gold film used as a drain or a source.

Comparative Example A silicon oxide film having a thickness of about 3000Å was provided on both surfaces of an n-type silicon plate (3.0 cm × 3.0 cm) having a conductivity of 6 S / cm and a thickness of 380 μm by a thermal oxidation method. Next, using a positive photoresist on one side, a pattern for forming a metal film that works as the source and drain lead wires (each effective area: 0.2 cm × 0.8 cm;
Draw a distance between both patterns: 6 μm), and then provide a chromium film with a thickness of 200 Å by the vacuum evaporation method, and further deposit a gold film on it 300 times.
Å After providing, the resist was removed to form a gold film acting as the source and drain lead wires. A lead wire was further applied to both of these lead wires with a paste, and a contact portion was fixed with an epoxy resin to obtain an element substrate.

Tetraethylammonium perchlorate (0.7g) and 2,
2'-Dithiophene (0.4g) in acetonitrile (100m
The reaction solution was prepared by bubbling nitrogen gas into l) for 30 minutes. Both gold films that will be the source and drain of the element substrate are used as working electrodes, and a platinum plate (1 cm x 2 cm) is used as a counter electrode.
These were immersed in the reaction solution using SCE as a reference electrode.
Using the working electrode as an anode, a constant current (30 μA) was passed between the counter electrode and a platinum plate for 5 minutes, and both gold films serving as a source and a drain and silicon oxide between the source and the drain were coated with polythiophene. Next, the potential of the working electrode was set to 0 V with respect to SCE with a potentiostat for 4 hours to adjust the polythiophenodoping amount. After that, 2 times with deoxygenated acetonitrile
After washing once, it was dried by blowing nitrogen gas and then completely dried in a vacuum. Thereafter, in the same manner as in Example 1, n-type silicon was made to act as a gate electrode.

As described above, a comparative FET device having the same structure as that shown in FIG. In this comparative example, 7 in FIG. 3 is a substrate / gate electrode made of n-type silicon.
Is a silicon oxide that functions as an insulating film, 4 is a polythiophene film that is a semiconductor layer, and 5 and 6 are chromium films covered with a gold film that functions as a source and a drain, respectively.

FIG. 4 is a characteristic diagram with respect to the gate voltage of the current flowing between the source and the drain when 30 V is applied between the source and the drain in the FET devices manufactured in Example 1 and the comparative example, and the horizontal axis is the gate voltage. The vertical axis represents the source-drain current. In the figure, 11 is a characteristic curve when the polypyrrole film is used as the drain in the device manufactured in Example 1, and 12 is a characteristic curve when the polypyrrole film is used as the source. 13 is a characteristic curve of the FET element produced in the comparative example.

As is clear from FIG. 4, in the FET element of the present invention, the source-drain current was largely modulated by the voltage applied from the gate, and the characteristics were remarkably improved as compared with the comparative example. Further, the element of this example was not deteriorated even after being left in the air for one month. Deterioration is not observed even when heated in air at 80 ° C., which often leads to improvement in characteristics.

〔The invention's effect〕

As described above, according to the FET element of the present invention, either the source or the drain is composed of the first π-conjugated polymer film, and the semiconductor layer which is the current path is further formed into the first π-conjugated polymer film.
By forming the second π-conjugated polymer film different from the conjugated polymer film, a device exhibiting stable and excellent electric characteristics can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view of an FET device according to an embodiment of the present invention, FIGS. 2 and 3 are sectional views of a conventional FET device, and FIG.
The figure shows 3 points between the source and drain of the embodiment of the present invention and the comparative example.
It is a characteristic view with respect to the gate voltage of the source-drain current when 0V is applied. In the figure, 1 is a substrate, 2 is a gate electrode, 3 is an insulating film,
4 is a π-conjugated polymer film serving as a semiconductor layer, 5 and 6 are source and drain electrodes, 7 is a substrate / gate electrode, 8 is a metal film serving as a lead wire from a source or a drain, and 9 is a source or a drain. Π-Conjugated polymer film serving as a drain, 10 is a metal film serving as a drain or source, and 11 and 12 are the π-conjugated polymer film which is a polypyrrole film as a drain or a source, respectively. A characteristic curve when used as a source, and 13 is a characteristic curve of the FET element of the comparative example. The same reference numerals in the drawings indicate the same or corresponding parts.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-60-261175 (JP, A) JP-A-55-130161 (JP, A) JP-A-58-12370 (JP, A) JP-A-61- 163658 (JP, A)

Claims (13)

[Claims] 1. An insulated gate field effect transistor for controlling the electric conductivity of a semiconductor layer, which is a current path between a source and a drain, by a gate voltage through an insulating thin film, wherein one of the source and the drain has a first π− A field-effect transistor comprising a conjugated polymer film, wherein the semiconductor layer comprises a second π-conjugated polymer film different from the first π-conjugated polymer film. 2. The π-conjugated polymer having at least one heterocyclic five-membered ring in at least one of the first and second π-conjugated polymer films. Field effect transistor. 3. The first and second π-conjugated polymers having a five-membered heterocyclic ring have the general formula (However, X is one of S and O atoms, R ₁ and R ₂ are one of —H, CH ₃ , —OCH ₃ , and —C ₂ H ₅ groups, and n is an integer.) The field effect transistor according to claim 2, characterized in that 4. The first and second π-conjugated polymers having a five-membered heterocyclic ring have the general formula (However, R ₁ and R ₂ are —H, —CH ₃ , —OCH.
_{3, -C} 2 one of a _{H 5} and _-OC 2 _{H 5} group, _{R 3} is _{-H, -CH 3, -C 2 H} 5, -C 3 H 7, and , N is an integer. ) The field effect transistor according to claim 2, characterized in that 5. The electric field according to claim 3, wherein the first and second π-conjugated polymers having a five-membered heterocyclic ring are polythiophene or poly (3-methylthiophene). Effect transistor. 6. The electric field according to claim 4, wherein the first and second π-conjugated polymers having a hetero five-membered ring are polypyrrole or poly (N-methylpyrrole). Effect transistor. 7. The electric field effect according to claim 2, wherein the first π-conjugated polymer film is polypyrrole and the second π-conjugated polymer film is polythiophene. Type transistor. 8. The first .pi.-conjugated polymer film is poly (3-methylthiophene), and the second .pi.-conjugated polymer film is polythiophene. The field effect transistor according to item 2. 9. The first to the second π-conjugated polymer films, at least one of which is obtained by an electrochemical polymerization method. A field effect transistor according to any one of items. 10. The first and second π-conjugated polymer films, at least one of which is doped, and the at least one of the first and second π-conjugated polymer films is doped. The field effect transistor according to. 11. The field effect transistor according to claim 10, wherein the at least one π-conjugated polymer film is electrochemically doped. 12. The polythiophene is obtained by an electrochemical polymerization method of 2,2'-dithiophene, according to any one of claims 5 and 7 to 8. Field effect transistor. 13. The field effect transistor according to claim 1, wherein the gate electrode is composed of one of p-type silicon and n-type silicon. .